The Function of Manures and Fertilizers 23 



must be studied from both points of view. Farmyard 

 manure is an example. It contains nitrogen, phosphoric 

 acid and potash, and possesses the power of improving 

 the physical character of soils. Lime, generally consid- 

 ered an indirect manure, may act in the capacity of a 

 direct manure upon soils sufficiently lacking in this ele- 

 ment. Other materials which act in both capacities under 

 peculiar conditions are magnesia salts, iron salts, basic- 

 slag, nitrate of soda and the like. 



UNAVAILABLE AND AVAILABLE PLANT-FOOD 



While, as already stated, any material containing 

 either one or all of the three essential constituents, nitro- 

 gen, phosphoric acid or potash, may serve as a direct 

 manure in the sense that it increases the potential fer- 

 tility of any soil, the value of the addition of such mate- 

 rials will depend not so much on the amount, as upon 

 the power that the plant may possess of acquiring it 

 and it is here that the difference between manures from 

 natural sources and those from artificial sources is again 

 quite manifest. That is, the fertility constituents in 

 natural manures are in large part combined with others 

 in the form of vegetable matter, and with the exception 

 of potash, they are, when in this form, largely insoluble, 

 and, therefore, cannot be used by the plants until after 

 decay begins. Whereas, in artificial manures, the con- 

 stituents may be not only soluble, but may be in a form 

 in which the plants can take them up immediately. 

 In the first case, the plant-food is said to be unavailable, 

 and in the second, it is said to be available. 



Nitrogen, one of the chief constituents of manures, 

 for example, exists in three distinct forms: (1) the or- 



